This invention in general relates to optical communications and in particular to devices and methods for modulating electromagnetic carrier waves in the optical region of the spectrum.
To convey information on an optical wave, some property of that wave has to be modulated or changed in accordance with the information and adopted coding system. The wave property may be intensity, phase, frequency, polarization, or direction, and the modulation format can be either analog or digital.
Devices for modulating optical carrier waves are commonly classified as either direct or external modulators. With the direct modulator, the output of the carrier source, such as a semiconductor laser, is made to vary in accordance with changes in the injection current which typically serves as the electrical analog of the information signal. External modulators accept a source output as an input and then change some property of the source output for transmission along the fiber trunk line.
Well known in the art are external modulators which rely on the electro-optical or acousto-optical properties of crystalline birefringent materials such as lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3). Here, a light wave passing through a crystal exhibiting the electro-optic effect is divided into an ordinary and an extraordinary wave by appropriate choice of the direction of polarization. These waves travel with different velocities. When a voltage is appled to the crystal, the velocities of the waves change, and intensity modulation can be produced by using the resulting phase difference.
In a typical modulator based on this scheme, the output of a laser is linearly polarized and arranged to make an angle of 45.degree. with the electrically induced birefringent axes of the electro-optic crystal. A voltage applied to the crystal then changes the propagation velocities of the ordinary and extraordinary rays causing the output from the crystal to be elliptically polarized. Variations in the modulating voltage thus cause corresponding changes in the output when the output is made to pass through an analyzer and thus the transmitted power from the analyzer also changes. A quarterwave plate is often inserted in the sysem to bias it to give optimum linearity to the modular characteristic.
Waveguide modulators utilizing this scheme are also known with the waveguide taking on the function of the electro-optic crystal. In one type of known waveguide modulator, the amount of coupling between two waveguides is changed by applying a modulating voltage. However, all of the known waveguides rely on relatively complex interferometric schemes to produce intensity modulation as their output. Thus, it is a primary object of the present invention to provide a simplified waveguide modulator and method for using it to provide intensity modulation of an optical carrier wave.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention, accordingly, comprises methods and use of the apparatus possessing the construction, combination of elements, and arrangement of parts exemplified in the detailed disclosure which follows.